Fix crash in AvrcpTargetService PlayerSettings

PlayerSettings can be updated before the native interface
is created, or right after it's destroyed. Added null
check to prevent crash.

Bug: 287537221
Tag: #feature
Test: ON/OFF BT while changing player settings
Test: m aosp_bramble-userdebug
Change-Id: Ie5e3cae39e2ca63189728209bf6605d07081e4ae